1. Field of the Invention
The present invention relates to a WDM (wavelength-division multiplexing) optical communication technology and particularly it relates to a technology for controlling an optical tunable filter needed to extract signal light with a specific wavelength from a plurality of segments of signal light multiplexed by a WDM method, each with a different wavelength.
2. Description of the Related Art
With the explosive increase of a data communication demand centered on Internet traffic, a large capacity and super-long haul backbone network is required. In that case, since a variety of user services are required, the realization of a highly reliable, flexible and economical network is also required.
By the progress of a wavelength-division multiplexing transmission technology and an optical amplification technology, recently transmission capacity and transmission distance have been remarkably increased and transmission line cost has also been reduced. However, if a conventional optical/electric conversion system or an electric switching system is adopted in order to follow a high-speed and large capacity signal and to increase the information processing capability of a network node, a node cost increases and the size of a node device also increases. In such a background, the development of an optical add drop multiplexer (OADM) and an optical cross connect (OXC) device, which replaces a large-scale electronic circuit with an optical device in an optical communication system in order to reduce the cost and size of a node, and performs a variety of processes of data in units of optical paths in an optical wavelength area, is expected.
In these devices, a lot of optical functional devices, such as an optical switch for turning signal light on/off, attenuating signal light and performing the 1×n switching of signal light, etc., a wavelength filter for distributing signal light for each wavelength and the like are used.
Of these optical function devices, an optical wavelength selecting device (hereinafter in this specification, called an “optical tunable filter”) that can select signal light with a desired wavelength from a WDM signal is a major key device for realizing such an OADM. Such an optical tunable filter includes an acousto-optic tunable filter (AOTF).
FIG. 1 shows a network configuration of an OADM node, and FIG. 2 shows a configuration of an OADM using an AOTF.
In FIG. 1, a network A, 1001 and a network B, 1002 are overlapped in node 1, and the OADM of node 1 drops three segments of signal light each with one of wavelengths λ1, λ4 and λ6, of WDM signal light consisting of six segments of signal light each with one of wavelengths λ1 through λ6 transmitted from the node n in network A, 1001, from network A, 1001, and transmits them to the node 2 of network B, 1002. Three segments of signal light each with one of wavelengths λ2, λ3 and λ5 are transmitted through the OADM to the node 2 of network A, 1001.
The network shown in FIG. 2 is a ring-structured network. The network comprises a running system (working system, indicated as system W), which is actually used, and a stand-by system (protection system, indicated as system P), which is used when system W fails. Since the respective configurations of systems W and P are the same, only the configuration of system W is described here. Although system W shown in FIG. 2 comprises three OADMs of OADM 1 (W), OADM 2 (W) and OADM 3 (W), they have the same configuration. Therefore, only the configuration of OADM 1 (W) is described here. An amplified spontaneous emission (ASE) suppression filter 2000 inserted in the middle of system W or P eliminates natural light noise (white noise) that is generated and accumulated by each amplifier existing in the ring-structured network.
WDM signal light transmitted to OADM1 (W) from OADM3 (W) is amplified up to a predetermined size by an amplifier 2001, and then is inputted to a photo-coupler (CPL) 2002. Although the signal light transmitted through the CPL 2002 is inputted to a rejection AOTF 2003, part of signal light demultiplexed by the CPL 2002 is inputted to an amplifier 2004. The signal light amplified by the amplifier 2004 is demultiplexed into a plurality of segments of signal light by a CPL 2005, and each demultiplexed are inputted to a drop AOTF 2006. The drop AOTF 2006 extracts signal light with a desired wavelength from the WDM signal light. The extracted signal light is designated as the drop output of the OADM 1 (W).
Then, this signal light from the OADM 1(W) is transmitted through an optical switch (OSW) 2100 for switching a system from W to P or vice versa, is inputted to a transponder 2200 and is demodulated.
The signal (ADD input signal) transmitted to the network from this node is, firstly, optically modulated by a tunable transponder 2300 to signal light with a predetermined wavelength. Then, the signal is inputted to the OADM 1 (W) through an OSW 2400. This signal light is multiplexed with signal light with a different wavelength transmitted from the node by a CPL 2007. The signal light multiplexed by the CPL 2007 is amplified up to a predetermined size by an amplifier 2008, and is inputted to a CPL 2009. Then, the signal light is inserted in the signal light transmitted without being blocked by the rejection AOTF 2003, of the signal light from the OADM 3 (W), and both are multiplexed. The signal light multiplexed by the CPL 2009 is amplified up to a predetermined size by an amplifier 2010 and then is transmitted toward the OADM 2 (W).
The signal light wavelength selecting characteristics of the rejection AOFT 2003 and the drop AOTF 2006 are controlled by a control unit (MC) 2011, based on information provided by a monitor/control system 3000 monitoring the operation of the entire network shown in FIG. 2.
As described above, an OADM node must have a function to insert (add) signal light with a desired wavelength in (to) light that is transmitted through a node, a function to drop (extract) signal light with a desired wavelength from light that is transmitted through a node and a function to block signal light transmitted through a node. Sometimes a function to collectively drop signal light is needed. This function is required in a node where two or more ring-structured networks or general networks are overlapped, and is used to transmit signal light consisting of a plurality of segments of signal light each with a different wavelength, from one network to another network. Sometimes a function to collectively block signal light is also needed. This function blocks signal light with a wavelength to be terminated, of multiplexed light transmitted through a node and a plurality of segments of signal light each with a wavelength that has the same wavelength element.
An OADM using an AOTF is, for example, disclosed by Japanese Patent Laid-open Nos. 11-218790, 11-289296 and 2000-241782.
In an OADM node, it is important for signal light with an arbitrary wavelength to be able to be demultiplexed and inserted for the purpose of the flexible operation of a network. For that purpose, it is necessary to apply the above-mentioned collective process to signal light with an arbitrary wavelength, and from this point of view, a device, such as an AOTF having a function to freely change a wavelength to be selected is useful. If signal light with a desired wavelength is selected and demultiplexed using this function to freely change a wavelength to be selected, the transmission center of the filtering characteristic of such a device must completely coincide with the desired signal light wavelength. If the transmission center wavelength does not coincide with a signal light wavelength, for example, in the drop (extraction) process, insertion loss increases or signal light with another wavelength is wrongly dropped, which is the fatal problem of an OADM device.
Generally, the wavelength of light emitted from a laser diode (LD), being a transmitting light source, fluctuates, and the transmission center wavelength of such a device for providing a filter characteristic also fluctuates due to a change with an elapse of time, an environmental change, a control error and the like. Therefore, in order to stabilize the operation of an OADM device, it is indispensable to detect a wavelength fluctuation error and to track it to perform feedback control. In the case of a drop process, in this tracking, dropped signal light is demultiplexed and designated as monitor light. Then, the monitor light is detected and its power value is controlled so as to become a maximum. Usually a method for controlling by checking only the magnitude of the receiving optical power of the monitor light, which is disclosed by Japanese Patent Laid-open No. 8-288932, is most economical and efficient.
However, since an AOTF has a selected wavelength fluctuation characteristic that is sensitive to ambient temperature, for example, a temperature change of only 1° C. leads to a 100 GHz difference in a selected wavelength, the optimal frequency of an RF (high frequency) signal to be applied to determine a wavelength to be selected is not uniquely related to the wavelength to be selected, and a wavelength to be selected varies due to fluctuations in ambient temperature. For example, although an AOTF selects a wavelength of 1,550 nm if an RF signal of 170 MHz is applied at ambient temperature of 25° C., it selects a wavelength of 1,558 nm if the same RF signal of 170 MHz is applied at ambient temperature of 35° C. Since the optimal frequency of an RF signal to be applied to an AOTF varies depending on ambient temperature, the AOTF sometimes wrongly selects signal light with a different wavelength in selecting a signal with one arbitrary wavelength from a WDM signal, and continues to select the wrong signal light by the earlier-mentioned tracking control.
If the wavelength band of a WDM signal is approximately 40 nm, such as only the first continuous set band, a so-called “C band” (center wavelength band: approximately 1,525 nm-1,565 nm, although there are a variety of definitions) or only the second set band, which is a continuous wavelength band longer than the first set band, a so-called “L band” (long wavelength band: approximately 1,570 nm-1,610 nm, although there are a variety of definitions), signal light with a desired wavelength can also be selected by actually measuring the optimal frequency of an RF signal to be applied to the AOTF to transmit light signal with a predetermined wavelength in such a band in the operating environment of the OADM, computing an RF signal with an optimal frequency needed to transmit optical signals with other frequencies in the band by interpolation and the like, based on the detected result, and applying a signal with the computed frequency. At this time, tracking control can also be started.
However, for example, if one optical tunable filter selects signal light from a band including both C and L bands, that is, 85 nm between 1,525 nm and 1,610 nm, in the above-mentioned method, there is a possibility of selecting a signal with a wrong wavelength.
More specifically, for example, when an AOTF selects signal light with one arbitrary wavelength from a WDM signal in which signals are arrayed at intervals of 100 GHz, signal light to be selected can be switched to adjacent signal light only by changing the frequency of an RF signal to be applied by approximately 100 kHz. For example, if an RF signal of 170.0 MHz must be applied to the AOTF in order to select signal light of channel 10, in order to select the signal light of channel 11, that is, an adjacent channel 100 GHz away from the channel 10, an RF signal of 169.9 MHz must be applied.
In other words, in a WDM signal including only a C or L band, the change frequency range of an RF signal to be applied to select a signal with one wavelength from each of all 44 channels, which are a plurality of segments of signal light arrayed at wavelength intervals of 0.8 nm (at frequency intervals of 100 GHz) is computed as follows:(44−1)×100 kHz=4.3 MHz
In order to control the AOTF so as not to actually select an adjacent channel signal light, the optimal frequency of an RF signal must be computed with the accuracy allowance of ±40 kHz. Therefore, an allowable error rate is 40 kHz/4.3 MHz=0.93% and the optimal frequency error rate must be approximately ±1%. In the above-mentioned method of computing the optimal frequency of an RF signal to be applied to the AOTF, this accuracy can be secured, but it almost reaches its limit.
However, if the AOTF selects signal light from a band including both C and L bands, the change frequency range of an RF signal to be applied to select a signal with one wavelength from this band is computed as follows, since a plurality of segments of signal light are arrayed at wavelength intervals of 0.8 nm in a band of 85 nm:85/0.8×100 kHz=approximately 10.6 MHz
The allowable frequency error rate of an RF signal to be applied for the first time with the accuracy of ±40 kHz must be 40 kHz/10.6 MHz=0.038%. In other words, the frequency of an RF signal to be applied for the first time must be obtained with an error rate of approximately ±0.4%. In the above-mentioned method of computing the optimal frequency of an RF signal to be applied to the AOTF, it is very difficult to secure this accuracy.